Ferromagnetic contacts can induce an exchange field into a nanodevice, but can also be used as a source of spin-polarized electrons and detector. In the latter case, so called tunneling magneto-resistance (TMR) devices are obtained when at least two contacts are ferromagnets (FM), the injector and detector contact. We have studied TMR physics in CNT-based quantum dots (QDs) early on.1 We have also found a pronounced exchange field that can lead to a splitting of the Kondo resonance is strongly coupled devices.2 Since ferromagnetic contacts turned out to be delicate, mostly because of their proneness to oxidation with an atomic-scale sensitivity of its magnetic pattern,3 we have studied single QD Coulomb resonances in the presence of FMs carefully and demonstrated that spurious signals due to e.g. the magneto Coulomb effect may not be effective in well prepared devices.4 In our most recent work, we have moved away from ferromagnetic contacts and explore the effect of an inhomogeneous stray field of nanomagnets in close proximity to CNTs and SNWs.5 This research is motivated by a recent theoretical proposal that demonstrate that a periodically varying magnetic field can mimic spin-orbit interaction. This synthetic spin-orbit interaction is interesting, since a simple array of ferromagnetic finger gates can induce large Rashba fields allowing to realize non-trivial topological states.

(a) Schematic of a NW with ferromagnetic side gates (FSG1 and FSG2) that result in a local stray field. The magnetization of the FSGs can be inverted by an external field B. (b) An actual device made from an InAs NW and Py nanomagnets. (c-d) Different measured traces for the magnetoresistance (MR) depending on the state of the NW QD. (c) is typically for the single QD regime, while d-e) require coupled QDs. One of the switches in the TMR-like curves in (e) is due to a ground-state change from a triplet to a singlet when lowering B.
  1. S. Sahoo, T. Kontos, J. Furer, C. Hoffmann, M. Graber, A. Cottet and CS, Nat. Phys. 1 (2), 99-102 (2005).
  2. L. Hofstetter, A. Geresdi, M. Aagesen, J. Nygard, CS and S. Csonka, Phys. Rev. Lett. 104 (24), 246804 (2010).
  3. S. Zihlmann, P. Makk, C. A. F. Vaz and CS, 2D Mater. 3 (1), 011008 (2016).
  4. J. Samm, J. Gramich, A. Baumgartner, M. Weiss and CS, J. Appl. Phys. 115 (17), 174309 (2014)
  5. G. Fabian, P. Makk, M.H. Madsen, J. Nygård, C. Schönenberger, A. Baumgartner, Phys. Rev. B 94, 195415 (2016).

Funding: SNF

Relevant papers (keyword: K_Ferro):


  • Magnetoresistance engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates
    G. Fábián, P. Makk, M. H. Madsen, J. Nygård, C. Schönenberger, and A. Baumgartner.
    Physical Review B, 94(19):195415, nov 2016. [DOI] arXiv:1608.07143

    We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The wire segment can be electrically tuned to a single dot or to a double dot regime using the FSGs and a backgate. In both regimes we find a strong MR and a sharp MR switching of up to 25\% at the field at which the magnetizations of the FSGs are inverted by the external field. The sign and amplitude of the MR and the MR switching can both be tuned electrically by the FSGs. In a double dot regime close to pinch-off we find {\it two} sharp transitions in the conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic contacts, with one transition near zero and one at the FSG switching fields. These surprisingly rich characteristics we explain in several simple resonant tunneling models. For example, the TMR-like MR can be understood as a stray-field controlled transition between singlet and a triplet double dot states. Such local magnetic fields are the key elements in various proposals to engineer novel states of matter and may be used for testing electron spin-based Bell inequalities.

  • Spin transport in fully hexagonal boron nitride encapsulated graphene
    M. Gurram, S. Omar, S. Zihlmann, P. Makk, C. Schönenberger, and B. J. van Wees.
    Physical Review B, 93(11):115441, mar 2016. [DOI] arXiv:1603.04357

    We study fully hexagonal boron nitride (hBN) encapsulated graphene spin valve devices at room temperature. The device consists of a graphene channel encapsulated between two crystalline hBN flakes: thick-hBN flake as a bottom gate dielectric substrate which masks the charge impurities from Si_{O2}/Si substrate and single-layer thin-hBN flake as a tunnel barrier. Full encapsulation prevents the graphene from coming in contact with any polymer/chemical during the lithography and thus gives homogeneous charge and spin transport properties across different regions of the encapsulated graphene. Further, even with the multiple electrodes in-between the injection and the detection electrodes which are in conductivity mismatch regime, we observe spin transport over 12.5 $\mu$m-long distance under the thin-hBN encapsulated graphene channel, demonstrating the clean interface and the pinhole-free nature of the thin hBN as an efficient tunnel barrier.

  • Role of hexagonal boron nitride in protecting ferromagnetic anostructures from oxidation
    S. Zihlmann, P. Makk, C. A. F. Vaz, and C. Schönenberger.
    2D Materials, 3(1):11008, feb 2016. [DOI] arXiv:1509.03087

    Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. In these systems, oxidation of the ferromagnetic materials poses an intrinsic limitation on device performance. Here we investigate the role of ex situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. The chemical state of the ferromagnets was investigated using x-ray photoemission electron microscopy because of its high sensitivity and lateral resolution. We have compared the oxide thickness formed on ferromagnetic nanostructures covered by hBN to uncovered reference structures. Our results show that hBN reduces the oxidation rate of ferromagnetic nanostructures suggesting that it could be used as an ultra-thin protection layer in future spintronic devices.


  • Fork stamping of pristine carbon nanotubes onto ferromagnetic contacts for spin-valve devices
    J. Gramich, A. Baumgartner, M. Muoth, C. Hierold, and C. Schönenberger.
    physica status solidi (b), 252(11):2496-2502, jul 2015. [DOI] arXiv:1504.05693

    We present a fabrication scheme called ‘fork stamping’ optimized for the dry transfer of individual pristine carbon nanotubes (CNTs) onto ferromagnetic contact electrodes fabricated by standard lithography. We demonstrate the detailed recipes for a residue-free device fabrication and in-situ current annealing on suspended CNT spin-valve devices with ferromagnetic Permalloy (Py) contacts and report preliminary transport characterization and magnetoresistance experiments at cryogenic temperatures. This scheme can directly be used to implement more complex device structures, including multiple gates or superconducting contacts.

  • Entanglement Detection with Non-Ideal Ferromagnetic Detectors
    P. Rozek, P. Busz, W. Klobus, D. Tomaszewski, A. Grudka, A. Baumgartner, C. Schönenberger, and J. Martinek.
    Acta Physica Polonica A, 127(2):493-495, feb 2015. [DOI]

    Entangled states are essential in basics quantum communication protocols and quantum cryptography. Ferromagnetic contacts can work as a spin detector, giving possibility of converting information about electron spin to the electric charge, and therefore, detection of entangled states with the electric current measurements is possible. Method of conrming entanglement with non-ideal detectors is presented, the impact of decoherence and noise on states and quality of entanglement is discussed. Entanglement witness (EW) operator method is compared with the CHSH inequalities approach. Required spin polarization for the EW is lower than for the CHSH inequalities. System with asymmetric spin polarizations of detectors was analyzed, including the CHSH inequalities and the EW method.


  • Optimized fabrication and characterization of carbon nanotube spin valves
    J. Samm, J. Gramich, A. Baumgartner, M. Weiss, and C. Schönenberger.
    J. Appl. Phys., 115:174309, may 2014. [DOI] arXiv:1312.0159

    We report an improved fabrication scheme for carbon based nanospintronic devices and demonstrate the necessity for a careful data analysis to investigate the fundamental physical mechanisms leading to magnetoresistance. The processing with a low-density polymer and an optimised recipe allows us to improve the electrical, magnetic, and structural quality of ferromagnetic Permalloy contacts on lateral carbon nanotube (CNT) quantum dot spin valve devices, with comparable results for thermal and sputter deposition of the material. We show that spintronic nanostructures require an extended data analysis, since the magnetization can affect all characteristic parameters of the conductance features and lead to seemingly anomalous spin transport. In addition, we report measurements on CNT quantum dot spin valves that seem not to be compatible with the orthodox theories for spin transport in such structures.

  • Entanglement witnessing and quantum cryptography with nonideal ferromagnetic detectors
    W. Kobus, A. Grudka, A. Baumgartner, D. Tomaszewski, C. Schönenberger, and Jan Martinek.
    Phys. Rev. B, 89:125404, mar 2014. [DOI] arXiv:1310.5640

    We investigate theoretically the use of non-ideal ferromagnetic contacts as a mean to detect quantum entanglement of electron spins in transport experiments. We use a designated entanglement witness and find a minimal spin polarization of η>1/3–√≈58 required to demonstrate spin entanglement. This is significantly less stringent than the ubiquitous tests of Bell’s inequality with η>1/2–√4≈84. In addition, we discuss the impact of decoherence and noise on entanglement detection and apply the presented framework to a simple quantum cryptography protocol. Our results are directly applicable to a large variety of experiments.


  • Kondo effect and spin-active scattering in ferromagnet-superconductor junctions
    H. Soller, L. Hofstetter, S. Csonka, Levy A. Yeyati, C. Schönenberger, and A. Komnik.
    Phys. Rev. B, 85(17):174512, may 2012. [DOI] arXiv:1204.6581v1

    We study the interplay of superconducting and ferromagnetic correlations on charge transport in different geometries with a focus on both a quantum point contact as well as a quantum dot in the even and the odd state with and without spin-active scattering at the interface. In order to obtain a complete picture of the charge transport we calculate the full counting statistics in all cases and compare the results with experimental data. We show that spin-active scattering is an essential ingredient in the description of quantum point contacts. This holds also for quantum dots in an even charge state whereas it is strongly suppressed in a typical Kondo situation. We explain this feature by the strong asymmetry of the hybridisations with the quantum dot and show how Kondo peak splitting in a magnetic field can be used for spin filtering. For the quantum dot in the even state spin-active scattering allows for an explanation of the experimentally observed mini-gap feature.


  • Permalloy-based carbon nanotube spin-valve
    H. Aurich, A. Baumgartner, F. Freitag, A. Eichler, J. Trbovic, and C. Schönenberger.
    App. Phys. Lett, 97:153116, oct 2010. [DOI] arXiv:1009.1960

    In this paper we demonstrate that permalloy (Py), a widely used Ni/Fe alloy, forms contacts to carbon nanotubes (CNTs) that meet the requirements for the injection and detection of spin-polarized currents in carbon-based spintronic devices. We establish the material quality and magnetization properties of Py strips in the shape of suitable electrical contacts and find a sharp magnetization switching tunable by geometry in the anisotropic magnetoresistance (AMR) of a single strip at cryogenic temperatures. In addition, we show that Py contacts couple strongly to CNTs, comparable to Pd contacts, thereby forming CNT quantum dots at low temperatures. These results form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance switchings in the tunneling magnetoresistance, which directly correspond to the magnetization reversals in the individual contacts observed in AMR experiments.

  • Ferromagnetic Proximity Effect in a Ferromagnet Quantum-Dot Superconductor Device
    L. Hofstetter, A. Geresdi, M. Aagesen, J. Nygård, C. Schönenberger, and S. Csonka.
    Phys. Rev. Lett., 104:246804, jun 2010. [DOI]

    The ferromagnetic proximity effect is studied in InAs nanowire based quantum dots strongly coupled to a ferromagnetic (F) and a superconducting (S) lead. The influence of the F lead is detected through the splitting of the spin-1=2 Kondo resonance. We show that the F lead induces a local exchange field on the quantum dot, which has varying amplitude and sign depending on the charge states. The interplay of the F and S correlations generates an exchange field related subgap feature.


  • Nanospintronics with Carbon Nanotubes
    A. Cottet, T. Kontos, S. Sahoo, H. T. Man, M. -S. Choi, W. Belzig, C. Bruder, A. F. Morpurgo, and C. Scönenberger.
    Semicond. Sci. Technol., 21:S78-S95, oct 2006. [DOI] arXiv:0703472

    One of the actual challenges of spintronics is the realization of a spin transistor allowing control of spin transport through an electrostatic gate. In this paper, we report on different experiments which demonstrate gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyse spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts.

  • Controlling spin in an electronic interferometer with spin-active interfaces
    A. Cottet, T. Kontos, W. Belzig, C. Schönenberger, and C. Bruder.
    Europhys. Lett., 74:320-326, mar 2006. [DOI] arXiv:0512176

    We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin-splitting of the wire resonant energies which is tunable with the gate voltage and the angle between the ferromagnetic polarizations. This property could be used for manipulating spins. In particular, it leads to a giant magnetoresistance effect with a sign tunable with the gate voltage and the magnetic field applied to the wire.


  • Electric field control of spin transport
    S. Sahoo, T. Kontos, J. Furer, C. Hoffmann, M. Gräber, A. Cottet, and C. Schönenberger.
    Nature Physics, 1:99-102, nov 2005. [DOI] arXiv:0511078

    Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them1, 2, 3. This approach is illustrated by the operation of the most basic spintronic device, the spin valve4, 5, 6, which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction7, 8. The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin- or energy-dependent3, the opposite can occur and the magnetoresistance is negative9. The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate10, 11. In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.

  • Electrical spin injection in multi-wall carbon nanotubes with transparent ferromagnetic contacts
    S. Sahoo, T. Kontos, C. Schönenberger, and C. Sürgers.
    Appl. Phys. Lett., 86:112109, mar 2005. [DOI] arXiv:0411623

    We report on electrical spin injection measurements on multiwall carbon nanotubes (MWNTs). We use a ferromagnetic alloy Pd1−xNixPd1−xNix with x≈0.7x≈0.7 which allows us to obtain devices with resistances as low as 5.6kΩ5.6kΩ at 300 K. The yield of device resistances below 100kΩ100kΩ, at 300 K, is around 50%. We measure at 2 K a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. The relative difference between the resistance in the antiparallel (AP)(AP) orientation and the parallel (P)(P) orientation is about 2%.


  • UHV compatible nanostructuring technique for mesoscopic hybrid devices: application to superconductor/ferromagnet Josephson contacts
    T. Hoss, C. Strunk, C. Sürgers, and C.~Schönenberger.
    Physica E, 14:341-345, may 2002. [DOI]

    We report on an ultra-high vacuum (UHV) compatible method for fabricating devices of sub-micrometer size by virtue of a non-organic evaporation mask of high thermal and mechanical stability. As an application we describe the superconducting properties of mesoscopic superconductor/normal-metal and superconductor/ferromagnet/superconductor hybrid structures. In particular, we report on the observation of the DC-Josephson effect in Nb/Cu/Co/Cu/Nb structures prepared in UHV. The Josephson coupling between the two superconductors through the very thin (5nm) magnetic and metallic weak link is confirmed by the magnetic field dependence of the critical current Ic, which displays a Fraunhofer-like interference pattern.